1. Field of the Invention
The present invention relates to an apparatus and a method for controlling electromagnetic radiation pulse duration.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device) or an IC. This pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (e.g., resist) using a beam of radiation. In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
In conventional lithographic apparatus, the beam of radiation may be provided by a radiation source (e.g., a laser or an arc lamp), and the radiation beam may be formed by or comprised of a plurality of radiation beam pulses. The radiation beam pulses may be generated by pulsing the radiation source or by selectively allowing or preventing the passage of a continuous beam of radiation. A certain exposure energy is associated with each of the radiation beam pulses, and the energy of a respective pulse may be calculated as the intensity of the radiation beam pulse integrated over the duration of the pulse. Thus, the energy of the radiation beam pulse may be increased by increasing the pulse duration and/or by increasing the intensity. Alternatively, the energy of the radiation beam pulse may be kept constant by varying the intensity by a certain amount and by varying the pulse duration by corresponding amount.
The beam of radiation may pass through lenses, gratings, masks, etc., or be reflected from mirrors or other reflective surfaces. The intensity of radiation beam pulses forming the radiation beam may be high enough to temporarily or permanently damage those surfaces that contact the radiation beam. The intensity of a given pulse may be sufficient to instantly damage the surfaces, or the cumulative effects of a plurality (e.g., millions or billions) of pulses may be sufficient to cause such damage. In some circumstances, such damage may be tolerable. However, in general, it is desirable to avoid damage to optical elements through which the radiation beam passes through or off which the radiation beam reflects.
Reduction or elimination of such damage increases the lifetime of the elements and also ensures that the performance of the elements does not degrade rapidly. One approach to reducing such damage is to reduce the intensity of the radiation beam pulses that form the radiation beam. However, if the intensity of the radiation beam pulses is decreased without a corresponding change in pulse duration, the total energy of a given radiation beam pulse will decrease. The decrease in total energy may not be desirable, as a radiation beam pulse may require a certain threshold energy in order to perform a certain function, such as applying a pattern to a photosensitive material. Therefore, in addition to reducing the intensity of the radiation beam pulses to reduce or eliminate damage to surfaces through which the radiation beam passes or off which the radiation beam reflects, it is also desirable to increase the duration (i.e., the length) of the radiation beam pulse to ensure that the total energy of the pulse remains unchanged and at or above the threshold value.
Conventional apparatus increase the duration of a radiation beam pulse by reflecting a portion of the radiation beam around a reflective optical circuit provided by mirrors. The reflected portion is then directed in the direction of travel of the non-reflected portion of the radiation beam. By carefully controlling parameters associated with the reflection of the portion of the radiation beam pulse, the reflected radiation beam pulse can be made to slightly lag and/or overlap with the non-reflected portion of the radiation beam pulse. The lag and/or overlap in the radiation beam pulses is such that the two radiation beam pulses, in combination, effectively serve as a single radiation beam pulse having an increased length (e.g., an increased duration).
These conventional apparatus succeed in reducing the intensity of a radiation beam pulse, while effectively increasing the length of the pulse. However, these conventional apparatus have at least one disadvantage. By reflecting the portion of a radiation beam pulse around an optical circuit using mirrors, losses associated with each respective reflection decrease the intensity of the reflected portion. As losses in intensity increase, it becomes more difficult to ensure that a radiation beam pulse with sufficient intensity is incident upon a target portion of the resist coated substrate at a later stage in the lithographic apparatus.
Therefore, what is needed is an apparatus and method that allows the pulse duration of a radiation beam to be extended without a corresponding loss in intensity.